The Rules of Go
(Wei Chi)

Equipment and Aim

Go pieces are black and white lens-shaped discs called stones.
The Go board can either be a flat table board or the more traditional
floor-board with legs (Go-ban). Either way, the board is simply
a grid of 19 x 19 lines, the pieces being placed upon the intersections
of the lines.

Go is a game of territorial capture - the primary objective is to encircle
as much territory as possible. In doing so, opposing stones may
be captured and the winner is the player at the end with the greatest
amount of territory and captured stones.

Basic Definitions and Play

The most essential terms to understand are "group" and "liberty".

A group of stones is any set of stones of the same colour that are connected
orthogonally. So three stones in a row along a line forms a group
because every stone sits orthogonally next to at least one other stone.
However 2 stones next to each other diagonally are not connected in any
way and so simply form two groups of one stone each. If a third
stone were to be added to the two diagonal stones so that it sat next
to both of them, however, a group of three stones would be formed.
Groups can get quite large and convoluted but the principle remains the
same - if a stone lies orthogonally next to another stone then both stones
are part of the same group.

Any empty point orthogonally adjacent to a group of stones is said to
be a liberty of that group. A single stone by itself in the middle
of the board therefore has 4 liberties, 1 in each of the 4 directions.
A group of 3 stones by itself in a line on the edge of the board has 5
liberties - 1 at either end and 3 towards the middle of the board.
And a group of 8 stones set in a square by itself has 13 liberties - 12
around the outside and 1 in the middle. But each stone laid by the
opponent next to a group reduces the number of liberties by 1. So
a single stone with opposing stones North, South and East of it has only
1 liberty.

Go is not only pleasing to the eye, the game itself is also beautifully
aesthetic in its simplicity. In fact, in essence there are really
only 3 rules to the game:

Starting with black, each player places stones on the board in turn.

When a stone is played so that it causes a group of opposing stones
to have no liberties, that group is captured.

A player cannot play a stone to a location if such a play would cause
a previous position to be repeated.

So a single stone is captured if the opponent places four stones on the
four orthogonal points surrounding it. And a group of 2 stones on
the edge of the board is captured by 4 enemy stones.

Eyes - the key to Go

An important point to realise is that a group of 8 stones set in a square
is difficult to capture because if the opponent places a stone in the
middle of the group, under most circumstances, that stone is immediately
captured by the surrounding group. Consequently, no player would
ever normally make such a play. The unoccupied point in the middle
of the group is an example of an "eye". An eye is any
empty point that is surrounded orthogonally by pieces of the same colour
- always difficult for an opponent to capture. However, eyes are
not impossible to take - the group of 8 stones can be captured by an opponent
who first occupies the 12 surrounding points. After this, the group
of 8 stones is vulnerable - if the player who owns it plays to the middle
of the group, the group of 9 stones would be immediately captured having
no remaining liberties. And this is the only situation where it
is legitimate for the opponent to play a stone to the middle since in
doing so, the last remaining liberty of the group is eliminated and the
group is captured. The stone just played would be left surrounded
by 4 liberties.

Derived from this is the key factor in Go defence - any group containing
two eyes is safe and can never be captured. This should be easy
to understand after a moments thought - in order to capture the group
all liberties must be eliminated and so both eyes would need to be occupied.
But since a stone played to either eye would immediately be captured,
it is impossible for both eyes to be occupied. QED.

In conclusion, eyes are useful and a group with 2 eyes is invulnerable.

Other terms

"Ko" is a local situation in which a position can be repeated
indefinitely. For an example, lay a white stone on the edge of
the board and a black stone three points away from it also on the edge.
Lay another white stone diagonally next to the first white stone in the
direction of the black stone. Finish the pattern symmetrically by
laying a second black stone diagonally next to the first black stone.
Now play a white stone on the edge next to both black stones.
This situation is Ko. A black stone played to the remaining point
in the middle captures the white stone but the white player can then play
another white stone back to the same spot capturing the black stone in
the same way and putting the position back to how it started. And
so on. Of course, due to the third rule above, the black player
would not be allowed to do this immediately after the capture of the black
stone - at least one stone must be played elsewhere before black could
play otherwise the a previous position would be repeated.

"Seki" is another local situation. This term applies
to an area into which neither player dare play because to do so would
cause the opponent to capture territory or stones.

To have "Sente" is to be in a position to make a move that
will force the opponent to take a counter-action. If a player with
sente makes the play in question and the opponent, instead of responding
in the predicted way, makes a different play with an even greater threat,
the opponent is said to have "assumed Sente".

Any group of stones that is under threat of imminent capture i.e. having
only one liberty left is said to be in "Atari".

A "dame" point is an empty point between territories. When
there is a dame point there is no benefit to either player. Dame points
are left alone until the end of the game and then ignored in scoring.

Starting

Go employs a simple and effective handicapping scheme. The weaker
player always plays black but also places an amount of stones onto the
board before the start of the game according to the amount of the handicap.
The board has nine highlighted intersections in a square shape marked
on the board called "star" points. The requisite number
of stones are placed the star points in the following way:

1 stone handicap - on a corner star point

2 stone handicap - on opposite corner star points

3 and 4 stone handicap - on 3 or 4 corner star points

5 stone handicap - 4 corner + 1 side star point

6 stone handicap - 4 corner + 2 opposing side star points

7 and 8 stone handicap - 4 corners + 3 or 4 side star points

9 stone handicap - all 9 star points.

Beginning the game in Go is both critical and very difficult to do well.
Players try to play stones far enough apart so that they form the beginnings
of territory encirclements but close enough so that they can be linked
up into groups should they come under attack. Initial stones
tend to be played near the corners - corners are the easiest places to
capture territory because they only have to be surrounded on two sides.
Good players will begin by positioning stones seemingly at random across
the board but in reality they are staking their claims to particular areas.
After this initial period, local skirmishes and larger battles will form
in areas of contention. Players need to be able to comprehend and
deal with all the smaller conflicts while never becoming distracted from
the overall picture of the war.

Finishing

Eventually, the players agree that no more stones can be played since
all territory is claimed and all local battles have been played to their
conclusion. Play continues until both players agree to this.
At this point the winner and the margin of victory is determined.
Essentially, each player scores the number of points of territory plus
the number of prisoners captured. The totals are unimportant - it is the
difference between them that is measured and it is customary to calculate
this in a special way.

Firstly, the stones on the board are moved around to form neater patterns,
easier for counting. Players do this by sliding stones from one
place to another taking care not to change the amounts of territory owned.
Some of the black stones will be moved to a different point within black's
territory so that the stone's new position reduces the territory by one
but the point freed up increases it by one to compensate. White
stones similarly. Once this repositioning has been done, all the
captured prisoners taken are laid down into enemy territory, again in
a neat way to make counting easy. This is done on the basis that
losing a point for a prisoner is equivalent to reducing by one the number
of points of territory held by the opponent.

Once the board and prisoners have been consolidated in this fashion,
the winner and the margin of victory can be quickly determined by a count
of the simplified territories.

Since it is generally recognised that black has an advantage by going
first, non-handicapped games are often decided as the best of two games
with players taking turns to play black. The margins of victory
are summed after both games have been completed to determine the winner.